any way to add coupon via incoming link?

In an attempt to encourage customers to move to my fancy new opencart 2 store, I wanted to put a link on my old website to the new website which includes a voucher code that would be automatically entered into the basket.

so a link would ideally be something simple like
www.newstore.com/index.php?voucher=1234

I cannot find any way of doing this. at the moment all I can say is:-
"please visit my new store but remember to enter coupon code 1234"
which is far from ideal.

I know it would be an empty cart but if I add products then a coupon manually and remove the product the coupon stays so it must retain the coupon even when the cart is empty

Anybody? I'm searching for the same solution. Want to make a mailing and activate the coupon immediately when clicking the link, no need to fill it in separately.